One feels justified in not forgiving. "Why should I forgive the bastard? He wronged me, and not just once!"
"Don't you need forgiveness? You must forgive because you yourself need to be forgiven for what you have done to others!"
"There is a part of me that can appreciate your fine Sunday School sentiment. But to survive in this ugly and brutal world I had to become tough and unfeeling; I had to learn how to be unjust and how to wrong people. And of course I had to practice on the weak and vulnerable if I was to make any progress and not be killed right away."